Do big carnivores practice birth control?

The grizzled wolf stalks from her rival’s den, her mouth caked with blood of the pups she has just killed. It’s a brutal form of birth control, but only the pack leader is allowed to keep her young. For her, this is a selfish strategy—only her pups will carry on the future of the pack. But it may also help the group keep its own numbers in check and avoid outstripping its resources. A new survey of mammalian carnivores worldwide proposes that many large predators have the ability to limit their own numbers. The results, though preliminary, could help explain how top predators keep the food chains beneath them in balance.

Researchers often assume that predator numbers grow and shrink based on their food supply, says evolutionary biologist Blaire Van Valkenburgh of the University of California, Los Angeles, who was not involved in the new study. But several recent examples, including an analysis of the resurgent wolves of Yellowstone National Park, revealed that some large predators keep their own numbers in check. The new paper is the first to bring all the evidence together, Van Valkenburgh says, and presents a “convincing correlation.”

Hunting and habitat loss are killing off big carnivores around the world, just as ecologists are discovering how important they are for keeping ecosystems in balance. Mountain lions sustain woodlands by hunting deer that would otherwise graze the landscape bare. Coyotes protect scrub-dwelling birds by keeping raccoons and foxes in line. Where top carnivores disappear, these smaller predators often explode in numbers, with potentially disastrous consequences for small birds and mammals.

Ecologist Arian Wallach of Charles Darwin University in Darwin, Australia, was searching for dingo tracks in the Australian outback when she began to wonder why dingoes and foxes have such different ecological impacts in Australia. “They’re cousins,” she says, but invasive foxes have expanded out of control and are blamed for wiping out many native marsupials, whereas social dingo packs are stable and help keep fox and kangaroo numbers in check. She wondered whether something about pack structure could explain the different roles played by small and large carnivores worldwide.

Wallach and colleagues gathered research on the life cycles of more than a hundred species of mammalian carnivores—from polar bears and panthers to skunks and stoats—and documented examples of large predators that apparently regulate their own numbers. Among the 73 best understood species they also tested how traits like parental investment, birth rate, and the number of females with young vary with body size. They found a size threshold at about 15 kilograms. Most smaller predators breed rapidly and have many offspring, whereas most larger species invest more time in each cub or pup. About half of large carnivores further regulate their numbers by only letting certain group members breed. Among wolves and hyenas, for instance, dominant females kill the pups of social subordinates. In many of these species, the whole group then raises the dominant animal’s pups communally.

It’s not news to wildlife ecologists that large carnivores give birth and mature at a leisurely pace or that they live complex social lives. But the pattern suggests that these natural population controls could be a defining feature of top predators, the authors argue online this month in Oikos. The hypothesis could have big consequences for conservation. Hunting and habitat loss now threaten 87% of large carnivores worldwide, the authors write, and conservationists should do more than keep track of population sizes. “Their ecology and their well-being ­[and] everything about them has to do with their social structure,” Wallach says. “And all we’re doing is counting them.”

Leading wildlife ecologists applaud the study for putting forward a surprising hypothesis and bringing together so much evidence from around the world. But they also caution that the jury is still out on the paper’s conclusions. “They’ve taken a step in the right direction,” says John Gittleman of the University of Georgia, Athens, who calls the new findings “very valuable.” However, he points out that the authors don’t actually test whether social factors or more traditional resource limits keep predator numbers stable over time. Van Valkenburgh also expresses concern that human persecution and fragmented habitats have disrupted carnivores’ social groups around the world. To find out what keeps populations constant, the researchers should study stable social groups over many years, she says. But to do that, they’d need a time machine.